The present invention relates to arc welding, and more particularly, to a method and device for igniting and stabilizing a welding arc of an arc welding apparatus.
Welding is a vital manufacturing technology in many industries. Welding processes are energy intensive as they require the production of high energy densities in order to create and move a pool of plasma. In most common welding methods, the energy coupling efficiency between the welding tool and a workpiece is twenty to thirty percent at best, depending upon the material and welding technology selected. Thus, significant economic benefits can be obtained if the coupling efficiency can be increased. Other aspects of the welding process, such as weld quality and productivity arc also of interest and can impact the economics of the welding process.
Because all of these factors are in some way dependent upon the energy density which is incident on the workpiece, much effort has been made to increase this quantity by developing novel technologies and welding tools.
Igniting a welding arc and keeping its burning stability is essential for any welding technology. Several methods of igniting a welding arc are described, for example, in the book "Welding Handbook (American Welding Society), vol. 2, 8th Edition, "Welding Processes", entitled "Arc initiation methods", pp.91-93, and include scratch or touch start, pilot arc start, high-frequency start, pulse start, high-voltage DC-current start, etc.
Scratch or Touch Start:
With the power supply energized, and the shielding gas flowing from a cup, the torch is lowered toward the workpiece until the electrode makes contact with the workpiece. The torch is quickly withdrawn a short distance to establish the arc, see, for example, SU Pat. No. 703265, entitled "Protective gas metal spot welding arc ignition".
The advantage of this method of arc initiation relies in its simplicity in operation for both manual and machine welding.
The disadvantage of touch starting is the tendency of the electrode to stick to the workpiece, causing electrode contamination and transfer of metal from the electrode to the workpiece.
Pilot Arc Start:
Pilot arc starting can be used with DC welding power sources. The pilot arc is maintained between the welding electrode and the torch nozzle. The pilot arc supplies the ionized gas required to establish the main welding arc.
As shown in FIG. 1, a pilot arc starting circuit 2 which is used for gas tungsten arc spot welding consists of a welding power supply 4 and a pilot arc power supply 6, which are connected with a tungsten electrode 8 and a workpiece 10. Tungsten electrode 8 is inside a coaxial anode ring 12 and insulated gas nozzle 14. A pilot arc 16 is powered by a small auxiliary power source, namely, pilot arc power supply 6 and is started by high-frequency initiation. See, in this respect, also European patent application EP 0753371 and SU Pat. Nos. 539703 and 1623846.
High-frequency Start:
High-frequency starting can be used with DC or AC power source for both manual and automatic welding applications. High-frequency generators usually have a spark-gap oscillator or thyristor oscillator that superimposes a high-voltage AC output at radio frequencies in series with the welding circuit. Such a circuit is shown in FIG. 2. A high frequency arc starting circuit 20 consist of a welding power supply 22 and a high frequency generator 24, which includes a spark gap oscillator 26 and an air core transformer 28, which are connected with an electrode 30 and a workpiece 32. Electrode 30 is placed inside a coaxial gas nozzle 34. The high voltage generated by high frequency generator 24 ionizes the gas present between electrode 30 and workpiece 32, and the ionized gas then conducts a welding current that initiates the welding arc. See, in this respect JP 10166145 and SU 1613263 patent documents.
High-frequency generator 24 is usually connected to circuit 20 in series and performs a breakdown of an air gap present between electrode 30 and workpiece 32.
A high-frequency discharge generates a conductive duct within the gap wherein, under certain conditions, there develops an arc discharge from the welding power source. Since the source open circuit voltage does not generally exceed 60 to 150 V, it is necessary to provide high conductivity of the discharge gap to generate an arc discharge, i.e., essentially heat the high-frequency discharge duct. Therefore, high-frequency generator 24 must be of sufficiently high power. In this case, high-frequency currents flow along the welding circuit, so the latter radiates in a wide frequency range, generating an intensive level of electromagnetic disturbances for radio appliances, electronic equipment and computers. These disturbances may be harmfull to people as well.
High-voltage pulses act in the welding circuit as long as high-frequency generator 24 is operated. Typical pulse shapes 36, 38 thereof are shown in FIGS. 3 and 4.
Since radiation from high-frequency generators affects people, environment, radioelectronic appliances, control equipment and computers, the application of welding arc igniting devices based on such generators requires high quality of manufacturing the latter, as well as adequate safety measures in operation which is, in some countries, regulated by the Government. So, in the U.S. the application of such arc igniting equipment is governed by regulations of the Federal Communications Commission.
Pulse Start:
In pulse start, application of a high-voltage pulse between a tungsten electrode and a workpiece ionizes a shielding gas and establishes a welding arc.
This method is generally used with DC power supplies in machine welding applications. In this respect, see, for example, U.S. Pat. Nos. 4,061,899; and 5,365,035, European patent applications EP 0149916; and EP 0586325, and patent documents JP 05084578; JP 09038771 and SU 1613263. The arc ignition is performed by high voltage pulses transmitted to the discharge gap by means of a high frequency transformer.
The breakdown of the discharge gap by high voltage pulses causes the appearance of high frequency oscillations in the welding circuit. This results in the appearance of disturbances, which are less than at a high frequency start but great enough to affect control, radio and electronic equipment, as well as computers.
High-Voltage DC Start:
High-voltage can be used with a DC-power source mainly for automatic applications. See, for example, U.S. Pat. No. 4,123,646 and JP 06179076, EP 0585068 and SU 448091.
In high-voltage DC start, arc ignition is performed by connecting a high voltage, low power, DC-power source (10 to 15 kV at 0.01 to 0.5 A) to a discharge gap. When an arc discharge is formed, the voltage decreases below that of the open circuit of the welding power source and the current flows now from the welding power source. This method creates a very low level of disturbances. Its main disadvantage is the need for protecting the welding power source and the user from the high voltage of the DC-power source. Therefore, this method is chiefly used for automatic welding, at very small currents, typically between 10 and 20 mA.
Methods and devices for igniting a welding arc are known, directed at lowering the electric field resulting from radiation disturbance and, at the same time, providing a stable arc burning, which is also the aim of the present invention. Such methods and devices have been developed by "Matsushita Electric Ind. Co. Ltd.", Japan, and are described in U.S. Pat. No. 5,582,751 and 5,773,792, and in JP 05261536, JP 06063745, JP 06182547, JP 06254679, JP 07009137, JP 07051853, JP 08118013, JP 08300147 and EP 0585068.
Most of the devices described in the above documents include a second DC-power source connected to the circuit of the welding power source and generating high voltage, a pulse of which is applied between the welding electrode and the workpiece, igniting an arc.
FIG. 5 shows an "arc welding unit" as described in JP 10166146 of "Matsushita Electric Ind. Co. Ltd.". This device comprises a welding circuit 40, which includes a welding power supply-transformer 41 supplemented with a current control element 42 and a processor 43, a DC high voltage circuit 44 supplemented with a resistor 45, which are connected to a non-consumable electrode 47 and to workpiece 48. An output of DC high voltage circuit 44 is superimposed in parallel with an output terminal between non-consumable electrode 47 and workpiece 48, and a single high voltage pulse from high voltage pulse generating circuit 49 is superimposed in series over the output terminal by a transformer 50 for coupling. In this device, a non-load voltage from circuit 40 must be no less than 100 V and no more than 1,000 V. A single high voltage pulse with a peak value of no less than 1,000 V is generated by high voltage pulse generating circuit 49. A welding current flowing from current control element 42 is detected by a current detecting circuit 51, and the output from circuit 44, as well as the output from high voltage pulse generating circuit 49 arc stopped.
The high voltage pulse creates a conducting duct in a gap present between electrode 47 and workpiece 48, wherein an arc discharge is developed from welding power source 41. Simultaneously with the application of a high voltage pulse, the conductive duct receives high DC-voltage from circuit 44 to at least partially suppress high frequency disturbances.
A drawback of the described device is that the voltage pulse generator operates during the whole period of arc ignition, transmitting high voltage pulses and creating a set of pulses of high frequency continuous oscillations in the transformer winding and the entire welding circuit.
High frequency currents flow along the welding circuit during the entire period of the high frequency pulse generator and the welding circuit emits electromagnetic radiation within a wide frequency range, creating an intensive level of electromagnetic disturbances dangerous to control, radio and electronic equipment, as well as to computers. In addition, within the entire period, the electrode and workpiece receive high (up to 1000 V) voltage, so special measures must be taken to protect the user.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and device for igniting a welding arc in an arc welding apparatus, which permit to essentially reduce the time required for arc ignition and lower the electromagnetic disturbances level to a minimum.